The existing versions of pressure sensors measuring differential pressure across a thin metallic diaphragm determines the differential pressure as a function of change in the electrical capacitance between the thin metallic diaphragm and a reference plane defined by a metallic planar member disposed parallel and closely adjacent to the thin metallic diaphragm, which electrical capacitance changes due to the lateral deflection of the thin metallic diaphragm resulting from the differential pressure loading on the thin metallic diaphragm. Such a method for measuring pressure or differential pressure relies on the measurement of the electromotive force drop in an electric circuit comprising the combination of the thin metallic diaphragm and the metallic planar member as a capacitive element included in the electric circuit, or by measuring the natural frequency of the electric circuit including the combination of the thin metallic diaphragm and the metallic planar member as a capacitive element of the circuit, which method relies on the measurement of the amplitude of an electrical signal, that is often subjected to drift due to aging of the electric circuit elements making up the electric circuit providing the electrical signal. As a consequence, the existing method for determining pressure or differential pressure is prone to errors arising from the drift in the characteristics of the electric circuit comprising the combination of the thin metallic diaphragm and the metallic planar member. It is well known fact that the phase angle difference between two alternating electrical signals is independent of drift in the amplitude of the two alternating electrical signals and consequently, determining a physical quantity as a function of the phase angle difference between two alternating electrical signals provides more reliable and accurate results compared with a method relying on the measurement of the amplitude of one of the two alternating signals. The present invention teaches a method for determining separation distance between two planes respectively defined by two thin metallic planar members as a function of phase angle difference between two alternating electrical signals, and its application to the construction of various embodiments of the pressure sensor that determines the pressure or differential pressure by measuring separation distance between a thin metallic diaphragm under the differential pressure loading and a reference metallic planar member.